A patent literature 1 (JP 2000-54833 A) disclosed a reducing agent supplying device that includes an ozone generator for generating an ozone, an air pump for blowing an air containing the ozone generated by the ozone generator, a fuel injector for injecting a fuel as a reducing agent, and a heater. A mixed air of the blown air and the injected fuel is heated by the heater, whereby the fuel is oxidized with oxygen in air, i.e., the fuel is reformed. The reformed fuel is supplied into an exhaust passage at a position upstream of a NOx catalyst to reduce NOx in the existence of the NOx catalyst, thereby purifying the NOx contained in exhaust gas. It should be noted that since the air contains the ozone, oxidation reaction of the fuel is accelerated, thereby improving a purifying rate for the NOx.
A patent literature 2 (US 2005/0011184 A) discloses a reducing agent supplying device includes a branch pipe through which a portion of intake air compressed by a supercharger flows from an intake pipe. The air taken by the branch pipe is blown by the air pump to be mixed with a fuel, thereby reforming the fuel.
However, there is a concern for the device according to the patent literature 1, as described below. When a pressure in the exhaust passage (i.e., exhaust pressure) is high during, for example, high engine load operation, the air pump may fail to supply the mixed air into the exhaust passage due to lack of a discharge pressure of the air pump.
In regards to the above-described concern, since the air compressed by the supercharger is blown by the air pump according to the device disclosed in the patent literature 2, the mixed air may be supplied into the exhaust passage even when the exhaust pressure is high. However, since the intake air may contain dust of, e.g., blow-by gas or internal EGR gas, there may be a concern that a failure occurs in the air pump due to the dust because the air pump according to the device of the patent literature 2 blows the intake air to the air pump. Especially, when the ozone is mixed with air to improve the NOx purifying rate, a failure may also occur in an ozone generator due to the dust.